Previous studies of reflexes mediated by joint afferent neurons may have employed inadequate or inappropriate means both for activating joint afferent neurons and for monitoring parameters of the movement of the joint. Consequently the nature of the feedback loop from joints to motoneurons innervating muscles acting at the joint is not well understood. The object of this research project is to evaluate reflex effects mediated by activity in joint afferent neurons, by employing precise adequate stimulation of the joint, and by monitorin both angular displacement of the joint and torque applied to the joint. The knee joint of the cat will be rotated with a mechanical stimulator and the monosynaptic reflex will be monitored in knee flexor or extensor motoneurons. Alterations of the magnitude of the monosynaptic reflex will be related to parameters of the stimulation of the joint. Initial results of this project indicate that joint reflexes are observed primarily at extreme angular displacement of the joint, and also that it may be more appropriate to characterize reflex changes by relating them to joint torque rather than joint angle. Therefore monosynaptic reflex alteration will be related in particular to joint displacement and joint torque. In order to relate reflex changes to activity in specific populations of afferent fibers, the time-dependent, adaptive nature of the response will be considered.